Skeletal muscle atrophy, the loss of muscle mass, is associated with removal of load-induced signaling either during disuse or under microgravity conditions. This atrophy is mediated by slowing/inhibition of growth signaling pathways and an increase in pathways associated with protein degradation (Goldberg, A. L. J Biol Chem 1969 244: 3223-3229; Jaspers, S. R. and Tischler, M. E. J Appl Physiol 1984 57: 1472-1479; Loughna et al. J Appl Physiol 1986 61: 173-179). This rapid loss of muscle mass and strength, especially in an aging population, represents a significant health problem.
As the primary response to the removal of load appears to be changes in either protein synthesis or degradation, a possible treatment strategy to decrease muscle atrophy would be to restore normal signaling for either or both of these processes. Investigation of growth pathway signaling has showed significant changes in the levels and activities of key signaling proteins following unloading (Gordon et al. J Appl Physiol 2001 90: 1174-1183; Hornberger et al. 2001 Am J Physiol Cell Physiol 281: C179-187; Hunter et al. 2002 Faseb J 16: 529-538; Mitchell, P. O. and Pavlath, G. K. Am J Physiol Cell Physiol 2001 281: C1706-1715). Overexpression of a constitutively active signaling protein, Akt, generated significant muscle hypertrophy and inhibited muscle atrophy associated with denervation (Bodine et al. Nat Cell Biol 2001 3: 1014-1019). However, upregulation of IGF-I, a potent activator of muscle hypertrophy (Barton-Davis et al. Proc Natl Acad Sci USA 1998 95: 15603-15607) did not inhibit the loss in muscle mass associated with hindlimb suspension (Criswell et al. Am J Physiol 1998 275: E373-379). This is indicative of unknown mechanisms blocking the IGF-I initiated cascade of growth pathways during disuse.
Rather than targeting the growth pathways, another potential therapeutic method to counter disuse atrophy would be to prevent increased activation of protein degradation. The loss of muscle protein, most notably myofibrillar protein degradation, is thought to occur primarily through activation of the ubiquitin-proteasome pathway (Taillandier et al. Biochem J 1996 316: 65-72; Tawa et al. J Clin Invest 1997 100: 197-203; Ikemoto et al. Faseb J 2001 15: 1279-1281) and causes declines in the force generating capacity of the muscle. However, two other proteolytic pathways, the Ca2+-dependent pathway (via calpains) and lysosomal pathway (via cathepsins B+L), have been associated with muscle atrophy and implicated in the initial proteolysis of the myofibrillar proteins (Tidball, J. G. and Spencer, M. J. J Physiol (Lond) 2002 545: 819-828; Tischler et al. Metabolism 1990 39: 756-763), though the role of these pathways in the loss of muscle protein is still unclear (Ikemoto et al. Faseb J 2001 15: 1279-1281). However, rats fed a 20% soy protein isolate diet were reported to have significantly higher calpastatin activity in gastrocnemius muscle than rats fed a casein diet (Nikawa et al. Nutrition 2002 18: 490-495, 2002). Thus, soy protein diets are suggested to prevent exercise induced protein degradation in skeletal muscle, possible inhibiting the calpain-mediated proteolysis (Nikawa et al. Nutrition 2002 18: 490-495, 2002). Studies investigating the effect of soy protein isolated on muscle atrophy caused by suspension hypokinesia have also been suggested to indicate that soy protein isolate causes a reduction in proteolysis of myofibrillar protein in skeletal muscles through reduction of calpain and proteosome activities, in consequence to ameliorate muscle atrophy (Tada, O. and Yokogoshi, H. J Nutritional Science Vitaminology (Tokyo) 48: 115-119, 2002).
Alternatively, recent work has suggested serine protease cascades may provide a mechanism for the initiation of protein degradation leading to muscle atrophy (Sangorrin et al. Comp Biochem Physiol B Biochem Mol Biol 2002 131: 713-723; Stevenson et al. J Physiol (Lond): 2003 2003.044701).
To date, however, there are no known oral pharmacological treatments for disuse atrophy and electrical stimulation to maintain muscle tone is still the primary method used to inhibit muscle loss during extended periods of inactivity.
Duchenne muscular dystrophy (DMD) is a degenerative disease that leads to progressive muscle weakness and atrophy. The disease results from the mutations in the gene encoding the cytoskeletal protein dystrophin. Dystrophin associates with a large complex of membrane-bound proteins, together the dystrophin glycoprotein complex (DGC), which is considered necessary for normal muscle cell membrane stability. The loss of dystrophin, and the associated DGC, results in compromised structural integrity of the muscle plasma membrane producing damaging cycles of muscle necrosis and regeneration.
To prevent DMD, it will likely be necessary to therapeutically replace either dystrophin itself or another protein capable of restoring appropriate function to the muscle. However, other therapies may greatly improve quality of life and diminish the severity of the disease, while the difficulties of protein replacement are overcome.
The loss of membrane integrity of muscles in patients with DMD is suggested to result in an increased influx of extracellular calcium, leading to activation of protein degradation pathways and stimulation of inflammatory processes. Thus, various attempts have been made to decrease the activation of the protein degradation pathway and/or inhibit stimulation of the inflammatory response.
For example, a correlation has been shown between calcium-dependent protease or calpain activity and dystrophic muscle and muscle necrosis. Overexpression of the transgene of calpastatin, a specific endogenous inhibitor of calpains was shown to reduce dystrophic pathology in mdx mice, a murine model for Duchenne muscular dystrophy (Spencer, M. J. and Mellgren, R. L. Human Molecular Genetics 2002 11(21):2645-55). Administration of the calpain inhibitor leupeptin has also been correlated with retention of myofiber size in the mdx murine model. (Badalamente, M. A. and Stracher, A. Muscle & Nerve 2000 23(1):106-11).
A trypsin-like protease designated as dystrypsin has also been reported to be markedly activated in the muscle microsomal fraction immediately before the onset of clinical signs in mdx mice. Camostat mesilate, a low-molecular weight inhibitor of trypsin-like proteases, including dystrypsin, has been suggested as a candidate drug for Duchenne muscular dystrophy (Sawada et al. Biological & Pharmaceutical Bulletin 2003 26(7):1025-7).
Direct injections of a mast cell stabilizer, cromolyn, have also been shown to increase strength in exercised mdx mice (Granchelli et al., Res. Commun. in Mol. Pathol. and Pharm. 1996 91(3): 287-96).
In addition, dietary supplementation with green tea extract, an antioxidant, was disclosed to reduce necrosis and decrease oxidative stress in mdx mice and cultured mouse C2C12 myotubes, respectively. (Buetler et al. American J. of Clin. Nutrition 2002 75(4): 749-53).
Currently, the only established treatment for muscular dystrophy, however, is the use of steroids such as prednisone and deflazacourt. These treatments slow the loss of muscle only slightly and produce significant side effects.